bas relief model from a 3d model/scene?
Generating nice heightmaps for bas reliefs:

1) Add a camera facing the model(s).

2) If there are multiple models, make sure the models are the same distance from the camera and have the same "thickness" (Y scale/size in the camera's local Y axis), make the latter 1 blender unit.

3) Set materials to black, shadeless.
Set background and ambient colors to white.

4) Set renderer to Blender Renderer.

5) Enable Mist.
This is where the magic happens. Have the mist appear where the closest polygons to the camera are, have it completely fade in when the farthest polygon from the camera is located.
If the camera is 4 units away from the closest point of the model, set Mist Start value to 4.0, and if the model has been scaled to 1 Blender unit in the Y axis, set the Mist Depth to 1.

A Bright/Contrast Node can be used in the compositing nodes to further adjust the rendered heightmap.

6) Important! Render in OpenEXR or another 32 bit format, otherwise the depth data stored in pixels won't look smooth when applied as a Displacement modifier later.

A model like this (download link in the question, in Blendswap),



Should render into something like this:


Use power-of-two resolutions for this image to save render time when used as heightmap.
Use a high resolution, such as 2048x2048.

Now, add a plane and subdivide it 10 times, or any other flat mesh with many polygons.
Add a Displacement modifier, and set its texture to the image you have generated.
Change the Strength value until it looks good for you.



You should end up with something like this:




To make the edges of your relief less sharp, blur the edges of the model in the heightmap image, or in Blender Compositor nodes when generating that heightmap.

Done.

There is probably a different way to do this besides using Mist. The point is to just have the closest points to the camera colored completely black and the farthest yet visible completely white, or the opposite way.